1. Field of the Invention
This invention relates to a MEMS (Micro-electro-mechanical System) actuator based on piezoelectric actuation such as a microswitch and a variable capacitor or the like.
2. Background Art
MEMS (Micro-electronic-mechanical System) actuators based on electrostatic actuation and piezoelectric actuation have a large variable capacitance ratio, a high Q-value and essentially high linearity, hence particularly they have a potential for RF (high frequency) device field and are highly expected for application to a high frequency switch and a variable capacitance capacitor.
However, this MEMS actuator has a problem called ‘stiction’. A movable beam of the electrostatic and piezoelectric MEMS actuator has a relatively small spring constant, hence when a charge is injected into a part of dielectric material constituting a MEMS structure body, electrostatic force derived from the injected charge is generated and the movable beam becomes easy to be stuck together with a fixed portion. This phenomenon is called stiction.
The electrostatic MEMS actuator includes an electrostatic actuation mechanism made of a movable electrode formed on the movable beam, a fixed electrode fixed on a substrate, and a dielectric film formed on a surface of the movable electrode or fixed electrode. By application of actuation voltage between the movable electrode (actuation electrode) and the fixed electrode, both electrodes are sucked in by electrostatic force to be actuated. However, when actuated, actuation voltage of a few tens volts is applied to the dielectric film with a normal thickness of about 0.1 μm to 1 μm, thus the dielectric film is subjected to a high electric field and charges are injected/trapped at interface or inside of the dielectric film in accordance with actuation time.
The injected charge exerts an effect similar to the externally applied actuation voltage on the electrostatic actuation mechanism, and hence significantly shifts the threshold voltage (pull-in voltage) for attaching the movable electrode to the fixed electrode and the threshold voltage (pull-out voltage) for releasing them. In more significant cases, the phenomenon called stiction occurs in which, even if the actuation voltage is reduced to zero, the electrodes remain stuck together and become inoperable, and is a serious problem in practice.
On the other hand, in the piezoelectric MEMS actuator, the movable beam includes the actuation mechanism made of the piezoelectric film sandwiched between electrodes and is actuated by relatively low voltage, and hence is less-sensitive to the charge injection into the piezoelectric film. However, the piezoelectric MEMS actuator also includes the movable electrode formed on the movable beam, the fixed electrode fixed on the substrate and the dielectric film formed on the surfaces of the movable electrode or the fixed electrode. When both electrodes are in contact with across the dielectric film, application of RF voltage having a large amplitude causes charge injection and trapping into the dielectric film, and in significant cases, the stiction phenomenon occurs in which, even if the piezoelectric actuation voltage is reduced to zero, the electrodes remain stuck together and become inoperable, causing a serious problem in practice.
JP-A 2006-140271(Kokai) discloses a structure provided with a dielectric film on both a movable electrode and a fixed electrode in a MEMS actuator.